It is impossible to state exactly the date of Harold's visit to Duke William in Normandy, although it is put at 1064. Probably Harold did make some kind of oath to William, most likely under compulsion. It is certain, however, that Harold helped William in a war with the Bretons.

On his return he married Ealdgyth, Griffith's widow, even though Edith Swan-neck, who had borne him five children, was still alive. In 1065 the Northumbrians rebelled against Tostig and Harold acquiesced in their choice of Morcar and Tostig's banishment. In January 1066 King Edward died. Harold, his nominee, was chosen king and crowned in Westminster Abbey.

Duke William lost no time in preparing for the invasion of England; and Tostig, after trying the Normans and the Scots, succeeded in drawing Harold Hardrada, King of Norway, to his side. In September the two reached the Humber and Harold marched to meet them. At Stamford Bridge he won a complete victory on 25 September 1066, Tostig and Harald Hardrada being among the slain. But four days later William landed at Pevensey. Harold marched southwards with the utmost dispatch and the two armies met at Senlac, about nine miles from Hastings. From nine in the morning, 14 November 1066, the English fought stubbornly until nightfall, when the pretended flight of the Normans drew them from their impregnable position and gave the Normans victory. Harold himself fell pierced through the eye with an arrow. His body was recognised by Edith Swan-neck and he was buried at Waltham.